Switching from flood to drip irrigation changes soil labile organic carbon in arid agricultural fields

Drip irrigation represents a key water-saving strategy in arid agricultural systems, but its influence on soil organic carbon (SOC) dynamics, particularly labile organic carbon (LOC) fractions, remains poorly understood. Here, we examined how converting flood to drip irrigation alters C stocks, LOC fractions, and associated microbial communities. The conversion from flood to drip irrigation decreased SOC while largely increasing soil inorganic carbon (SIC), ultimately enhancing total C stocks in the whole soil profile. Specifically, drip irrigation reduced particulate organic carbon (POC) by 47 %, 54 % and 31 % in the 0-20 cm, 20-40 cm and 40-60 cm layers, respectively; but increased light fraction organic carbon (LFOC) by 66 % in subsoil (40-60 cm). Although enzyme activities remained largely unaffected, drip irrigation enhanced microbial biomass (Gram-positive bacteria, Gram-negative bacteria, and fungi) throughout the soil profile and increased microbial carbon use efficiency (CUE) by 71 % and 8 % in topsoil (0-20 cm) and subsoil layers, respectively. The observed negative correlations between CUE and LOC components demonstrated that microbial carbon utilization efficiency did not primarily drive LOC accumulation. Our results demonstrate that converting flood to drip irrigation depleted SOC and POC reserves, highlighting the need for targeted mitigation strategies in drip irrigation systems.